One of the representative methods for various color recording methods may be a recording method using an ink jet printer, that is, an ink jet recording method. This recording method involves producing small droplets of ink, and attaching these small droplets to various record-receiving materials (paper, film, clothes and the like) to perform recording. In this method, the method is characterized in that since the recording head and the record-receiving material are not brought into direct contact, the process is quiet with less noise generation, and size reduction or an increased speed can easily be achieved. Therefore, the method has rapidly become popular in recent years, and an extensive growth in use of the method is expected.
Conventionally, aqueous inks prepared by dissolving a water-soluble coloring matter in an aqueous medium have been used as the inks for fountain pens, felt pens and the like and as the inks for ink jet recording. In these aqueous inks, water-soluble organic solvents are generally added so as to prevent clogging of the inks at the pen tips or ink discharge nozzles. Furthermore, these inks are required to have advantages such as the production of recorded images of sufficient densities (high print density), no occurrence of clogging at the pent tips or nozzles, satisfactory drying properties on record-receiving materials, less bleeding, and excellent storage stability. Further, the water-soluble coloring matters used therein is required to have high solubility particularly in water, and high solubility in the water-soluble organic solvents that are added to the ink. In addition, the images thus formed are required to have image fastness properties such as water resistance, light fastness, gas fastness and moisture resistance.
Among these, the above-described term gas fastness is a resistance to the phenomenon that oxidizing gas such as ozone gas present in air acts on the coloring matter within a recording paper and changes the color of a printed image. In addition to ozone gas, examples of oxidizing gases having this kind of action include NOx and SOx. However, among these oxidizing gases, ozone gas is considered as a main causative substance which accelerates the phenomenon of discoloration and fading of ink jet recorded images. For this reason, among the gas fastness, particularly ozone gas fastness tends to be regarded as most important. In an ink-receiving layer provided on the surface of exclusive ink jet paper of photographic image quality, materials such as porous white inorganic substances are frequently used for the purpose of speeding up drying of the ink, and reducing the bleeding at high image quality. In the images recorded in such a recording paper, significant discoloration and fading of the recorded image due to ozone gas is observed. Since the phenomenon of discoloration and fading due to an oxidizing gas, is characteristic to ink jet images, an enhancement of ozone gas fastness has been one of the most important problems to be solved in the ink jet recording method.
In order to expand the field of use in those recording (printing) methods using ink in the future, it is strongly desired to further enhance light fastness, ozone gas fastness, moisture resistance and water resistance in the ink composition used in ink jet recording and the colored body colored by the ink composition.
Inks of various colors have been prepared from various coloring matters; however, among them, a black ink is an important ink that is used in both of mono-color and full-color images. However, it has many difficult points technically to develop a coloring matter having a neutral hue between the dark color gamut and the light color gamut, and having high print density, and further having less dependency of the hue on the light source and exhibiting good black. Thus, although many researches and developments have been performed, those having sufficient performances are still few. For the reason, it is generally performed to prepare a black ink by combining multiple, versatile coloring matters. However, the preparation of an ink by mixing multiple coloring matters has problems such as 1) variation of the hues depending on the medium (record-receiving materials) and 2) particularly, increase of discoloration by decomposition of the coloring matters by light or ozone gas, in comparison to adjustment of an ink with a single coloring matter.
A black ink composition for ink jet to render a printed matter to have good various durabilities is proposed in, for example, Patent Documents 1, 2 and 3, and the like. This ink composition is a greatly improved ink composition having good image fastness properties of a printed matter. However, use of these ink compositions as an ink in a single color has not been applied to a neutral black printed matter, and have not sufficiently satisfied further demands of the market requiring high image fastness properties. In addition, Patent Documents 4 and 5 disclose a technique of combining various inks to obtain a black ink composition. However, there is no disclosed ink composition satisfying high fastness properties and color development properties, whereby to satisfy demands of the market.    Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2009-84346    Patent Document 2: PCT International Application, Publication No. 2009/069279    Patent Document 3: PCT International Application, Publication No. 2005/097912    Patent Document 4: PCT International Application, Publication No. 2007/077931    Patent Document 5: Japanese Unexamined Patent Application (Translation of PCT Application), Publication No. 2009-512737